Re: [PATCH v3] diff-delta: use safe integer arithmetic

From: Srujith Poojary <poojarysrujith@gmail.com>
Date: Oct 25, 2026, 19:21 UTC
Message-ID: <CABh_KWhE1eWYt60TvcN_7sAe6ORut2YH7a5Zmu2zLobv=VSefg@mail.gmail.com>
In-Reply-To: <xmqqmrs1y7kj.fsf@gitster.g>

On Sun, Oct 25, 2026 at 11:40 PM Junio C Hamano <gitster@pobox.com> wrote:

Show 19 quoted lines

>
> Srujith Poojary <poojarysrujith@gmail.com> writes:
>
> > Thank you for the detailed correction.
> >
> > I have expanded the commit message to include the step-by-step
> > breakdown for both the unpacked and packed index allocations,
> > detailing the caps on hsize (2^26) and entries (2^28) alongside struct
> > padding on 32-bit systems. I also corrected the phrasing regarding
> > (hsize + 1) being passed as an argument to st_add3().
>
> It seems that the text of v4 did not "detail" anything like that;
> you just parroted the conclusion, the final numbers I gave you,
> without giving any hint for the readers to validate the math.  The
> readers who are interested in validating are forced to figure out
> how many bytes your calculation assumes each struct type involved
> consume---and if they find that your conclusion is wrong, they have
> no clue where in your computation things went wrong.
>

Hi Junio,

Thank you for pointing that out. You are completely right—I should have explicitly detailed the struct sizes and exact code bounds instead of quoting summary numbers.

I have updated the commit message in v5 to explicitly state:
- The byte sizes of each struct on 32-bit platforms (12/16 bytes for
unpacked_index_entry, 4 bytes for hash pointers).
- The exact origin of the bounds from code: entries capped by
(0xffffffu / RABIN_WINDOW = 1,048,575) and hsize capped at 1,048,576.
- Why st_mult/st_add/st_add3 are used to guard against size_t wrap-around.

Best regards, Srujith